add stub appveyor.yml to satisfy CI #149

Job	Run time
Unit test (oldstable, macos-latest)	32s
Cross (stable, ubuntu-latest)	1m 34s
Unit test (oldstable, windows-latest)	1m 4s
Golang-CI Lint (ubuntu-latest)	18s
Unit test (stable, macos-latest)	36s
Unit test (oldstable, ubuntu-latest)	37s
Unit test (stable, windows-latest)	1m 13s
Unit test (1.17.x, ubuntu-latest)	19s
Unit test (1.17.x, windows-latest)	41s
Unit test (1.17.x, macos-latest)	44s
Unit test (stable, ubuntu-latest)	33s
	8m 11s

Provide feedback